Months of tension erupted into open hostility last night when Karen Lewis, president of the Chicago Teachers Union, called the work stoppage in the nation’s third-largest district. The teachers’ contract expired in June, and negotiations broke down over health benefits and job security. About 5 p.m., thousands of strike supporters gathered at school headquarters in the Loop business district waving placards and chanting; talks with the union, which represents 26,000 teachers and other workers, continued.

“Don’t take it out on the kids of Chicago if you have a problem with me,” Emanuel said at a church providing child care. Children stood behind him, while strikers in red T-shirts chanted outside. One held a sign picturing the mayor with horns.

The strike leaves Emanuel, a 52-year-old former chief of staff to Obama who is aiding groups advancing his re-election, caught amid conflicting interests.

Unions have been among the strongest supporters of Democrats nationwide, and are crucial to Obama’s prospects. At the same time, some of Emanuel’s wealthiest supporters have pushed for changes to the city’s education system. They include Penny Pritzker, a Board of Education member who is part of a Chicago real-estate dynasty, and Bruce Rauner, who is chairman of GTCR Rauner LLC, which manages equity investments.